Define a new critical data builtin policy to allow measuring
early kernel integrity critical data before a custom IMA policy
is loaded.

Update the documentation on kernel parameters to document
the new critical data builtin policy.
Signed-off-by: NLakshmi Ramasubramanian <nramas@linux.microsoft.com>
Reviewed-by: NTyler Hicks <tyhicks@linux.microsoft.com>
Signed-off-by: NMimi Zohar <zohar@linux.ibm.com>